1. Field of the Invention
The present invention relates to a high frequency module, and particularly to a high frequency module having a high-frequency signal lines through which high-frequency signals are transmitted.
2. Description of the Related Art
Recently, development of high frequency modules for high frequency waves, such as microwaves and milliwaves, have been in progress to deal with higher frequencies and higher output. As a result, the high frequency modules are mounted on an increasing variety of communication devices, and are widely used. In particular, the high frequency module is also used in a high output amplifier which is used in a mobile communication terminal device such as a cellular phone, and which is used in a base station as a power amplifier module having a substrate in which a matching circuit and a semiconductor element are integrated.
In such a high frequency module, a microstrip line is used in addition to various kinds of lines, such as a coplanar line and a coaxial line, as a high-frequency signal line which serves as a transmission line (refer to, for example, JP-A 2004-153368(KOKAI)). The microstrip line is mounted on a line substrate formed of a dielectric material, and is pulled out of the high frequency module using a lead wire. Also, the line substrate is mounted on a base substrate. Generally, the base substrate is made very thin, and has a thickness of about one tenth of the wavelength of the high-frequency signal so as to obtain good high frequency properties by preventing wraparound of the high-frequency signal when the high-frequency signal passes the lead wire.
However, in general, the base substrate and the line substrate are joined to each other by surface contact (bimetal structure). For this reason, when the thickness of the base substrate is smaller, the mechanical strength of the base substrate is reduced. Thereby, the warpage of the base substrate occurs. In a case where the high frequency module is mounted on a heat release and grounding member for heat loss and grounding, this warpage causes a gap between the base substrate and the heat release and grounding member. Thereby, thermal resistance increases, and high frequency properties are deteriorated.